Conduction of Heat in Liquids. 



31 



place through thin layers of the liquid, and thus in interpreting the 

 results, the conditions at the surfaces limiting the liquid layer are of 

 primary importance. It has been assumed by each observer that con- 

 tiguous surfaces of any two media are in all circumstances at the same 

 temperature. This, however, is contradicted by some high authorities, 

 so it would seem important to have independent results based on 

 experiments in which the liquid layer is of considerable thickness. 



It should also be noted that in methods employing thin layers the 

 temperature varies so rapidly in passing from one surface to the other 

 that the liquid forms a by no means very homogeneous medium. This 

 is the more important because experiments indicate that the conduc- 

 tivity of most if not all liquids increases rapidly as the temperature 

 rises. 



The following experiments were carried out in the Cavendish 

 Laboratory at the suggestion of Professor J. J. Thomson, to whom I 

 am much indebted for suggestions as to the form of the apparatus 

 and the methods to be employed. 



Two series of experiments were made with different apparatus. In 

 the earlier series it was found that the apparatus was too large to be 

 conveniently worked, and few results of a satisfactory nature were 

 obtained. In the second series the apparatus was much reduced in 

 size, though otherwise closely resembling that first employed. It will 

 thus be sufficient to describe the second form and supply data as to the 

 size of the first. 



The liquid was contained in a wooden tub with vertical sides, 

 19 '15 cm. in diameter, which was carefully fitted up by the mechanic 

 at the Cavendish Laboratory. Not far below the rim and at equal 

 distances apart were fixed three conical wooden pegs. The axes of 

 the pegs formed parts of radii of a horizontal section of the tub, pro- 

 jecting inwards from the cylindrical surface to a distance somewhat 

 exceeding 2 cm. The pegs supported a flat, thin-bottomed dish of 

 tin-plate, 14*85 cm. in diameter, whose base was thus maintained 

 horizontal. The liquid was poured into the tub till it reached the 

 base of the dish. The liquid surface being strictly horizontal, it was 

 easy to judge by the eye whether the dish was so also ; if not the tub 

 had to be adjusted till it was so. It was then advisable to stir the 

 liquid to make sure that no air bubbles remained clinging to the dish. 

 The bottom of the dish was about 5*2 cm. above that of the tub. 



The method required the temperature to be measured at a known 

 depth below the liquid surface. This end was secured by measuring 

 the electrical resistance of a fine straight platinum wire 6"6 cm. long, 

 which was supported at a depth of 2*61 cm. below the surface by two 

 small trestles of glass. These were fixed in the bottom of the tub and 

 projected upwards. The platinum wire was drawn tight over them, 

 each end being fused to a much thicker piece of copper wire, so that 



